In the art of wet processing, and more particularly, in the electronics industry in which sheets such as printed circuit boards, printed circuit film, foil and the like undergo various processing steps, preparatory to placing electronic components on the sheets. During such processing, the surfaces of the articles are generally etched to provide paths for current flow. Such etching is generally done after a masking operation, during which portions of the sheets are provided with predetermined zones, in which etching is desired to be avoided, leaving the uncoated layers to be etched, then rinsed, dried, etc.
Before or after application of a mask to desired surfaces of the sheets, the sheets are provided to the wet processing system from a source at which they are fabricated, which will generally occur at some time prior thereto, with the sheets often being stored in the interim until needed for use.
In order to protect the sheets, in this interim period after their manufacture, and prior to their being subjected to the wet processing, the boards have generally been coated with protective layers, to protect them during handling or storage of the same, to avoid scratching the surfaces thereof.
Such protective coatings are often oxide coatings, and most often are chromates. One such chromate is zinc chromate (ZnCrO.sub.4). Sometimes the coating will additionally, or instead, be of a lacquer or the like substance.
In order to remove such coatings, the sheets, may be mechanically worked by means of various grinding, abrading, scrubbing, brushing, honing operations or the like. However, with more sophisticated developments in electronics, the printed circuit sheets tend to be increasingly thinner, such that the removal of protective coatings by mechanical abrasions techniques without damaging the sheet, or without rendering unduly thin the subjacent metal of the sheet, generally copper, becomes extremely difficult.
Accordingly, mechanical removal of protective layers has its limitations.
The chemical removal of protective coatings has also been tried, such as by subjecting the sheet to a spray of acid, such as hot hydrochloric acid. However, such has also been found to have its limitations, and to sometimes be undesirable, because such acids often attack the subjacent generally copper substrate.
Some removal techniques have involved the use of an alkaline solution to remove the oxide coatings, but such also has its limitations because, depending upon which oxides are present, uncertain amounts of the coatings are removed, even though such alkaline solutions generally do not attack the substrate, particularly if the same is copper.
Such prior art techniques have therefore had deficiencies, as respect to removal of protective coatings from printed circuit sheets.